The present invention relates to methods and apparatus for broadband frequency characterization of radio wave signals and particularly for broadband frequency characterization of frequency hopping radio wave signals.
Frequency hopping is a form of spread-spectrum signaling where, for short instances of time, relatively narrowband signals are transmitted as short bursts with the carrier frequency for each burst tuned to a different one of a set of carrier frequencies than the ones of the carrier frequencies used for the previous burst and the next burst. The sequence of frequencies that is used for a sequence of bursts is known as the hopping sequence. The carrier frequency transmission at any particular instant of time for one burst is therefore different than the carrier frequency transmission at the previous instant of time for the previous burst and similarly is different than the carrier frequency transmission at the next instant of time for the next burst. While the bandwidth for any particular burst may be narrow, the bandwidth for the whole set of frequencies in the hopping sequence can be very large. Typically a frequency hop system hops over a bandwidth many times the bandwidth of the individual hop signal bandwidth. Bluetooth for example has a 1 MHz signal bandwidth and hops over 80 MHz. Some military radios have 25 kHz signal bandwidth with thousands of hop frequencies covering over 50 MHz. The frequency hoppers in use today hop over at least 8 times the bandwidth of the signal bandwidth.
Frequency hopping systems with changing frequency transmissions have a number of advantages over the fixed frequency transmissions of non-hopping systems. If a particular hop frequency, in the set of frequencies used in a hopping sequence, happens to include a frequency that is regularly occupied by another interfering radio signal, the frequency hopping system detects the occupied status and functions to retransmit the burst of data at a different frequency. Also, the frequency hopping system detects the regularly occupied frequencies for any particular installation and reestablishes a hopping sequence that excludes the occupied frequency from the set of frequencies in the hopping sequence.
Frequency hopping systems are more secure than fixed frequency systems because the interception of frequency hopped signals is significantly more difficult than interception of fixed frequency signals, particularly when the hopping sequence is not known in advance. If a communication protocol is intended to be secure, such as in military and other secure environments, the hopping sequence and other protocol, specification and standards information is not published and is changed from time to time to support secure operation.
In any environment, the characterization of radios and radio wave signals for frequency hopped systems is difficult because they operate and function over broad bandwidths and because each burst at a particular frequency is of relatively short duration. The characterization of signals for frequency hopped systems is even more difficult when done in a secret environment where the protocol, specification, standards, hopping sequence and other characterizing information is not fully known in advance. A secret environment is common since manufacturers and users of frequency hopping systems often wish to maintain their protocols, specifications, standards and hopping sequences confidential and unpublished.
In order to avoid the difficulties of characterizing frequency hopped systems, some analysis systems require the test radio to be put in a “hop-in-place” mode where the carrier frequency is the same for all bursts, that is, the frequency does not hop. Such “hop-in-place” systems eliminate the burden of dehopping the signal so that the analysis is much easier. However, such systems do not fully test the parameters that relate to or are affected by hopping and hence cannot fully characterize frequency hopping radios. Examples of such “hop-in-place” systems are the Agilent 89441A system with the Bluetooth® module and the Aeroflex RCTS-001 Radio Test Set.
In order to avoid the difficulties of characterizing frequency hopped systems when frequencies actually hop, some analysis systems require that the next hop in a hopping sequence for the test radio be known in advance by the analysis system. When the next hop in a hopping sequence is known in advance, such systems synchronize the frequency hopping of the analysis system with the frequency hopping of the test radio. In order to know the next hop in a hopping sequence in advance, analysis systems use the tuning information present in the radio signal messages of the test radio to derive each next hop frequency in the hopping sequence. Therefore, the next hop frequency is known in the analysis system before the hop actually occurs. Using the information from the radio signal messages, the analysis system synchronizes with the radio signal. In this manner, the frequency of the analysis system's local oscillator (LO) follows the hopping sequence in advance and thus can be used to dehop the radio signal being analyzed. In order for such systems to work, analysis systems are built to fully implement the radio signal protocols, specifications and standards, including the hopping sequence, for each radio to be analyzed. Such systems are expensive to build because many different radio signal protocols and standards exist. Further, such systems can only be used by those having full access to the signal protocols and standards including the hopping sequences. For many radio systems, however, for security and other reasons, such information is often not widely available. Examples of analysis systems that employ prior knowledge of the hops in a hopping sequence are the Agilent E1852B and Tektronix CMU 200 systems and the Marconi system (see U.S. Pat. No. 6,195,383). These Agilent and Tektronix test systems are designed to test the frequency hopping Bluetooth networks. They have internal Bluetooth modules that allow the test systems to become part of the network. These internal modules produce the carrier frequency values as they communicate with the radio under test external to the test system.
Because the “hop-in-place” analysis systems provide limited information, they are not fully adequate for the communication industry. Because the “known-in-advance” analysis systems have cost disadvantages and do not perform well for analyzing radios that are not operating within the specification range for the protocols and standards, they are not fully adequate for the communication industry.
Accordingly, in order to meet the demands of the communication industry, improved methods and apparatus are needed for analyzing frequency hopping signals, and other broadband signals, when the hopping sequences and other signal protocols and standards are not relied upon prior to analysis.